Sinn Fein has said loyalists who plan to mount a "Harryville-style" picket at St John the Baptist's Church on the Garvaghy Road in Portadown, Co Armagh, will be guilty of blatant sectarian provocation.
Loyalist sources have said the picket will begin within the next fortnight. It will be organised in protest at the banning of the Orange Order from the Garvaghy Road.
The Sinn Fein Assembly member for the area, Dr Dara O'Hagan, said: "We are consistently lectured by the Orange Order about their commitment to civil and religious liberty for all. At the same time it appears that their supporters are planning to prevent people practising their religion at their local church.
"This act is clearly designed to increase the climate of fear within the Catholic community in Portadown. It must not be allowed to take place. No protesters should be allowed within half-a-mile of the Catholic church."